Communications technology based on the use of optical radiation as an information carrier medium has rapidly reached a point of large-scale commercial utilization, augmenting and often supplanting more traditional approaches based on wire or microwaves. Commercial viability of optical communications may be attributed to the convergence of a number of developments such as, most prominently, the perfection of low-loss optical fibers, of light sources in the form of semiconductor light-emitting diodes and lasers, and of high-speed photodetectors. Particular attention is paid in the following to laser light sources and their suitability for use in optical communications systems.
Prominent among communications lasers are those commonly designated as distributed-feedback (DFB) lasers as predicted on feedback induced by periodic variations in optical characteristics along an active medium. In this respect see, e.g., U.S. Pat. No. 3,760,292, issued Sept. 18, 1973 to H. W. Kogelnik et al., and U.S. Pat. No 3,868,589, issued Feb. 25, 1975 to S. Wang. In an alternative laser design, a structure of periodic variations of optical properties (e.g., in the form of a grating) is placed beyond rather than alongside an active medium, thereby acting as a Bragg reflector. For example, as disclosed in U.S. Pat. No. 4,386,838, issued Sept. 1, 1981 to J. P. Huignard, a device may include a semiconductor active portion which is optically coupled to a polymeric reflector portion. Or, as disclosed in U.S. Pat. No. 4,464,762, issued Aug. 7, 1984 to K. Furuya, a silicon dielectric compound is suitable for the fabrication of a distributed Bragg reflector (DBR) coupled to a semiconductor active medium.
Preferably, as disclosed by Y. Abe et al., "GaInAsP/InP Integrated Laser with Butt-jointed Built-in Distributed-Bragg-reflection Waveguide", Electronics Letters, Vol. 17 (1981), pp. 945-947, field profiles and refractive indices of the active and reflector portions are matched in the interest of providing good coupling and low interface reflectivity.
Suitability of a laser for communications purposes depends on a number of criteria such as, e.g., significant suppression of nonlasing modes; in this respect see, e.g., J. M. Hammer et al., "Single-wavelength Operation of the Hybrid-external-Bragg-reflector-waveguide Laser Under Dynamic Conditions", Applied Physics Letters, Vol. 47 (1985), pp. 183-185. Among other requirements for optical communications are narrow linewidth for coherent applications, low chirp when dispersive transmission media are employed, and precisely selected wavelength in wavelength-multiplexed systems.